Yes, I have done this with multiple machines. No problems.
Not sure about the "maxing out" part though. Mac Pro 6,1 pulls are a good source of 4GB udimms, but you should look elsewhere for higher capacity modules. Most PC3-16000R or PC3-16000E modules will work fine in both machines at the processors' max speeds.
Edit: changed PC3-10600 (1333 MHz) to PC3-16000 (1866 MHz).